Investments Summary of Available for Sale (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Jun. 30, 2015	Mar. 31, 2015
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	$ 1,868,461	$ 1,719,843
Available-for-sale, gross unrealized gains	1,652	15,011
Available-for-sale, gross unrealized losses	(3,086)	(474)
Available-for-sale, estimated fair value	1,867,027	1,734,380
Government agency bonds [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	799,662	741,780
Available-for-sale, gross unrealized gains	494	676
Available-for-sale, gross unrealized losses	(1,150)	(200)
Available-for-sale, estimated fair value	799,006	742,256
Municipal bonds [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	41,142	41,552
Available-for-sale, gross unrealized gains	9	155
Available-for-sale, gross unrealized losses	(765)	(9)
Available-for-sale, estimated fair value	40,386	41,698
Auction rate securities [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	9,825	9,825
Available-for-sale, gross unrealized gains	0	0
Available-for-sale, gross unrealized losses	0	0
Available-for-sale, estimated fair value	9,825	9,825
Time deposits [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost		506
Available-for-sale, gross unrealized gains		0
Available-for-sale, gross unrealized losses		0
Available-for-sale, estimated fair value		506
Corporate bonds and debt [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	1,017,832	924,818
Available-for-sale, gross unrealized gains	1,149	2,376
Available-for-sale, gross unrealized losses	(1,171)	(265)
Available-for-sale, estimated fair value	$ 1,017,810	926,929
Marketable equity securities [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost		1,362
Available-for-sale, gross unrealized gains		11,804
Available-for-sale, gross unrealized losses		0
Available-for-sale, estimated fair value		$ 13,166

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=51813488&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (aa)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

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